1. Field of the Invention
The present invention relates to a bellows which is used for, for example, vehicle suspension devices, gas springs, accumulators, and vacuum apparatuses, and relates to a manufacturing method for the bellows.
2. Description of the Related Art
Austenite stainless steel (Japanese Industrial Standard (=JIS)·SUS304) is often used as a material for the above metallic bellows since the austenite stainless steel is easily processed. Alternatively, precipitation hardening stainless steel is used as a material for the above metallic bellows so as to improve durability of the metallic bellows. In producing a metallic bellows, first, both longitudinal sides of a stainless steel plate having an appropriate thickness are brought into contact with each other and are connected by arc welding, so that a material pipe is produced. Next, the material pipe is cut at a predetermined length and is then subjected to cold rolling using, for example, spinning working so as to have a predetermined thickness of a bellows. After that, the pipe is subjected to solution heat treatment by heating to a solution temperature thereof, and is then subjected to plastic working such as hydraulic bulging so as to be formed into a bellows. In a case in which precipitation hardening stainless steel is used, precipitation hardening heat treatment is further performed.
In a case in which a metallic bellows is dynamically used so as to act as, for example, a spring, setting is performed so as to stabilize the loading property of the bellows after precipitation hardening heat treatment, and surface working is then performed by providing compressive residual stress to a surface of the bellows so as to improve durability thereof. The surface working is performed by shot peening in which metallic particles are shot at a surface of the bellows at high speed or by honing in which glass beads are used instead of the metallic particles (for example, paragraphs 0015 to 0022, Table 1, and FIG. 1 in Japanese Unexamined Patent Application Publication No. 2001-304412).
Austenite stainless steel such as SUS304 is better in view of workability (for example, paragraphs 0006 to 0013, and FIGS. 1, 3, 4 and 6 in Japanese Unexamined Patent Application Publication No. 2003-172449). However, a metallic bellows made of SUS304 is low in material strength, whereby the design maximum stress thereof is 850 MPa. In order to meet requirements of miniaturization and light weight of bellows in recent years, high strength materials may be used. An example of the high strength material is SUS631 of precipitation hardening semi-austenite stainless steel. When SUS 631 is used as a material for the bellows, the design stress thereof is 1100 MPa, which is about 1.3 times as large as that of a case in which SUS304 is used (for example, lower left and lower right of column 3 in Japanese Unexamined Patent Application Publication No. 1-184225).
However, when fatigue test is performed on the bellows made of SUS631, the fatigue strength of the bellows made of SUS631 is lower than that of SUS304 as shown in FIG. 8. Generally, when the material strength is increased, the workability is reduced due to decreased elongation of the material. In particular, in the metallic bellows, since bulging having large reduction rate is performed, the fatigue strength may be reduced due to material defects such as micro-cracks generated in the bulging.